The handheld seismometer is
designed to illustrate concepts of seismometry (sensing and recording the
vibration or shaking of the ground generated by propagating seismic waves) and
to be used as a simple seismometer for educational demonstrations and
activities in seismology.The handheld
seismometer is designed to be as visual and inexpensive as possible to best
meet these objectives.Modifications
could improve the sensitivity and performance characteristics of the
seismometer, probably at the expense of simplicity and cost.

Theory:

Most seismometers detect
ground motion using a mass which is suspended in some fashion by a spring.When the ground moves (say in an up and down
or vertical motion), the seismometer's frame also moves, but the mass tends to
remain relatively steady because of inertia.The relative motion of the mass with respect to the frame (and thus the
ground) is then converted to an electrical signal (voltage variations with
time) using a capacitor plate, a galvanometer or a magnet and coil
assembly.Seismometers are usually
sensitive to one component (direction) of ground motion (vertical or
horizontal) and thus three components are used to completely characterize the
seismic signals.A typical vertical
component seismometer design is illustrated in Figure 1.Difficulties in understanding the theory and
operation of such a design and of working seismometers are caused by:1) the usual complexity of a modern,
sensitive electromagnetic seismometer, 2) the difficulty of experimenting with
the construction and components of the sensor, 3) the confusion caused by the
fact that, although it appears in demonstrations (by physically moving the mass
up and down) that it is the mass that is moving, it is actually the frame that
moves and the mass tends to remain steady, 4) difficulty in understanding the
electromagnetic sensor (transducer) theory and operation, and, 5) seismometers
are usually expensive instruments enclosed in a case to improve performance and
protect the mechanism, making it difficult for students, educators and other
non-seismologists to obtain and experiment with the instrument.

The handheld seismometer
solves some of these difficulties by being simple, visible and inexpensive to
build.Principles of seismometry,
including electromagnetic sensing using a magnet and coil, simple harmonic
motion of the spring and mass system, damping and recording the seismic signal,
can be easily demonstrated.One of the
most effective demonstrations, is to simply hold the seismometer in one's hands
and then "shake it" by moving the base up and down and noting the
output.The seismometer can also be used
on a tabletop for manual experimentation.

The magnet and coil sensor
operates by measuring the change of the magnetic field by the amount of voltage
produced in the surrounding coil.The
sensor characteristics (primarily its sensitivity) are controlled by the
strength of the magnet, the number of turns of wire in the coil and the
distance of the magnet from the coil.Once the sensor is constructed, it is easy to demonstrate that the
electrical output of the sensor is approximately proportional to the velocity
of motion of the mass with respect to the coil.Alternatively, a more effective way to introduce the electromagnetic sensor
used in many seismographs is to use a magnet and coil apparatus (Figure
2).One can use the magnet and coil to
illustrate the generation of a current or voltage in the coil that is caused by
the motion of a magnetic field.Connect
the ends of the magnet wire from the coil to the simple seismometer amplifier
circuit and the Vernier Serial Box Interface or to a digital multimeter.Hold the magnet inside the coil.Note that if the magnet is held steady, the
output of the electrical signal is near zero (or a constant).When the magnet is moved up and down, a
voltage is produced.When the magnet is
moved rapidly, a larger voltage is generated.Note that moving the magnet up and down within the coil produces alternating
positive and negative signals representing the relative motion of the magnet as
compared to the coil.

Constructing the Handheld Seismometer:

Construction details and
parts are provided on the attached diagram (Figure 3) and list.Figure 4 is a photo of a completed handheld
seismometer.

Using the Handheld Seismometer:

The seismometer can be
connected to a recording device in a variety of ways.The amplifier circuit and Vernier SBI (see
option number 4 in the "viewing the output signal" section on the
parts list, Table 1) is recommended because it provides a visual output of a
seismogram on a computer.One can even
save the seismograms for later use and for comparison of different shaking
sequences.The computer display also
aids in developing understanding of data acquisition and analysis
concepts.An effective sequence is:

1.Hold the magnet
assembly inside the coil to show that the output signal is not directly related
to displacement (position in a vertical direction) but to relative motion
(actually approximately proportional to velocity) of the mass with respect to
the coil.Alternatively, use the
separate magnet and coil assembly and procedure (described above) to illustrate
the electromagnetic coupling.

2.With the mass
suspended from the spring (as shown in Figure 3) and no oil in the container
(no damping), cause the mass to move up and down to illustrate simple harmonic
motion.Note that here is a
"natural frequency" of the oscillation.Measure the period (1/frequency) of oscillation
and note the sinusoidal characteristics of the signal.

3.Repeat the steps
in number 2 with oil in the container for damping.This is the standard operating set-up for the
seismometer.

4.Illustrate an
incoming seismic wave and movement of the base (normally attached to the
ground) by vibrating the seismometer vertically in your hands.Experiment with a "single pulse"
input and a longer duration, more complex input (similar to the series of
seismic waves produced by an earthquake source and wave propagation effects
through the Earth from the source to the seismograph).See attached sample output of the handheld
seismometer, Figure 5.

The handheld seismometer could be placed on the ground or
floor to record small ground motion from nearby footsteps, or local seismic
noise (such as generated by traffic) or weight drop experiments.However, several factors reduce the
effectiveness of the handheld seismometer in this application.These factors include:the relatively low sensitivity of the
seismometer; the high frequency of typical nearby sources, particularly on a
concrete floor; the damping of the seismometer (which has positive effects as
described above) which reduces the sensitivity, particularly for high frequency
(³
10 Hz) signals; and a high frequency resonance (unstable oscillation) of the
suspended spring-mass system which is caused primarily by horizontal shaking.

To illustrate the recording of different size signals
(different levels of ground motion), hold the seismometer in your hands and
move it very slightly upwards.Next,
move it moderately and then significantly upward.The three different sized motions of the
seismometer base should be visibly distinct and will produce three different
sized output pulses.Note that the
seismometer output is proportional to the size of the input shaking.Because of this characteristic, the output of
the seismometer (a seismogram) can be used to determine the level of ground
shaking of the location of the seismometer.Stronger shaking will be generated by larger magnitude earthquakes.The level of shaking at a particular location
is also controlled by the distance of the seismograph from the earthquake.

The handheld seismometer can also be used to illustrate common
data recording principles.Note that
while holding the handheld seismometer as steady as possible, there is still
some shaking and therefore a small signal on the output record.Because there is no specific source
(intentional shaking of the seismometer or earthquake–generated ground motion)
during this time interval, we consider the output signal to be background
"noise." Note that a small tap
(a specific source of shaking) on the base of the seismometer may not produce a
signal that is visible above the noise that is being continuously
recorded.Because of the presence of
background noise on the seismograph, small signals or signals from distant
earthquakes are sometimes not visible on the seismogram because amplitudes
(levels of shaking or height of the signal on the seismogram) of the signal may
be smaller than the noise level.Furthermore,
the presence of background noise encourages seismologists to find relatively
quiet (low background noise) sites for locating seismographs.Sometimes ground shaking is too large to be
accurately recorded by a seismograph.This situation can easily be illustrated with the handheld seismometer
by moving the base rapidly by a large impulse upwards.If the motion is large enough, the suspended
mass will hit the top or bottom of the container or the recording equipment
will "saturate" because a voltage that is too large to be recorded by
the interface (analog to digital converter) or recording software is
generated.In this case, the top (or
bottom, or both) of the recorded signal will be truncated or
"clipped."When the signal is
clipped, the maximum amplitudes of the ground shaking will not be accurately
recorded, and the signal will be distorted in the time interval where clipping
occurs.Both the noise and clipping
problems, from too small or too large levels of ground motion, occur commonly in
actual recordings of earthquakes by seismographs.

5.Demonstrate the
components of a seismograph:mass and
spring that sense the ground motion; magnet and coil sensor that converts the
relative motion into an electrical signal that can be recorded; amplifier;
digitizer (computer interface, Vernier SBI); recorder (computer and
software).Note that an absolute time
base, usually provided by an accurate chronometer or clock synchronized with
universal time, is missing from this assembly.Such an absolute time base is usually included in a research-quality
seismograph to provide earthquake monitoring capability.

Viewing the Output
Signal:

A visual record or
permanent recording of the output can be accomplished by a variety of methods
including:

1.Directly to a
strip chart recorder.

2.Directly to an
oscilloscope.

3.Connect to a
digital voltage meter (VOM; also called a multimeter or volt meter).Digital display will display movement and
alternating positive and negative voltages.

4.An inexpensive
and effective method to view the output signal of the magnet and coil assembly
or the handheld seismometer is to utilize an amplifier circuit and an analog
(with needle and dial scales; use 5 volt DC scale) multimeter.The amplifier circuit (Figures 6 and 7; Table
2) can be constructed (in about one hour after all parts are obtained; wire
cutter/stripper and small needle-nose pliers are useful, although not required
for assembly) using readily available parts for less than $20.An analog multimeter (or VOM or volt meter)
can be purchased for less then $20.The
output of the magnet and coil or handheld seismometer is very visible from
movements of the needle on the multimeter (the DC offset feature of the
amplifier circuit centers the needle, when there is no motion and therefore no
signal from the magnet and coil, at about +2 volts).Motion of the magnet relative to the coil
causes the needle to move back and forth on the multimeter scale.Small velocity motion of the magnet produces
small deflections of the multimeter needle.Large velocity motion produces large deflections of the needle.Photographs of the amplifier circuit are
provided in Figures 8 and 9.

(1)The
Handheld Seismometer has been designed primarily for demonstration purposes and
to be as simple and inexpensive as possible and to use readily-available
parts.Its performance and appearance
could be improved with a more complex and expensive design.Many parts could be substituted.

(2)Other,
relatively weak springs will also work, but may require an adjustment in the
mass and length of the long PVC pipe pieces.The Servalite #59 spring is about 12 cm long and 1.5 cm diameter and has
a 1.5 cm attachment ring at each end.The spring has a 3 cm stretch with a 200g mass suspended.Spring available at Ace Hardware and other
hardware stores.

(3)I
apologize for the non-metric units.However, wood and hardware items available in the United States
are generally manufactured and described by customary units.

(4)Nearly
any small block of wood will work as long as it is thick enough to accept the
mounting screws, wide enough to provide stability, and about 25 cm long to allow
mounting of the PVC 'T' joints sufficiently far apart.

(5)Other
containers could be substituted with suitable design changes.Diameter of the Klear Stor 236 ml container
is appropriate for the mass, constructed from washers, that is used here.The clear container allows the mass and
magnet to be seen during shaking.The
5.5 cm diameter allows 200-400 turns of fine magnet wire to be wound for the
coil.Apply ~ 3 cm wide strip of
two-sided tape to container before winding magnet wire.Be sure to scrape the coating off of the ends
of the magnet wire so that electrical connection is possible.One can also add small alligator clips to the
ends of the magnet wire for convenience in connecting to recording device.

(6)One-half
inch PVC (plastic) pipe is inexpensive and available at most hardware
stores.It is 1/2" inside
diameter.CPVC pipe is also available
but is more expensive because it is designed for hot water, and it is also more
flexible, making it less suitable for use in the frame.The outside diameters of 1/2" PVC and
1/2" CPVC pipe also differ, so they are not interchangeable.

(7)Washers
are labeled by the inside diameter.Two
of the 1/4" washers are used for mounting the PVC 'T' joints to the wood
base.Drill a 1/4" hole in the PVC
'T' joint.

(8)Straws
have slightly different diameters.Most
plastic straws available at fast-food restaurants have a diameter slightly
greater than 1/4" and thus fit over the threaded 1/4" eyebolt.The straw provides for relatively low
friction if the eyebolt comes in contact with the edges of the hole in the
plastic cap during shaking.

Table 2.Parts List – Amplifier and DC Offset Circuit

Parts for Amplifier and DC Offset Circuit
(parts are available at Radio Shack or electronics suppliers.If you have difficulty finding the ICL 7660
IC, contact Mouser Electronics, 800-346-6873, www.mouser.com).

Figure
1.Schematic diagram of a
seismometer.Ground motion causes the
base and frame of the seismometer.The
mass, suspended by the spring and boom, tends to stay in one place because of
inertia.The relative motion of the base
as compared to the mass is recorded as the output of the seismometer.

Figure
2.Photo of magnet and coil used to
illustrate electromagnetic coupling used in a seismometer.A small plastic container wrapped with magnet
wire (100 or more turns; place two-sided tape on the outside of the container
to help hold the magnet wire) forms the coil.Solder alligator clip leads to the ends of the wire.The magnet (one or more circular magnets) is
attached to a bolt for convenience in holding the magnet and moving the magnet
inside the coil.For the coil assembly
shown here, a 118 mL (4 oz.) container (Rubbermaid "Servin' Saver")
was used.Fine gauge magnet wire was
wrapped on the outside of the container.Rubber bands were used to keep the wire from unwinding.

Figure
5.Sample output from the handheld
seismometer using the amplifier interface circuit (see Simple Seismograph
activity), Vernier software SBI interface and Logger Pro display software.The input signal (shaking of the seismometer
held in one's hands) was a single impulse (one sudden upward motion of the
seismometer's base) followed by a few seconds of shaking the seismometer up and
down.

Figure
6.Amplifier and DC offset circuit
diagram.

Figure
7.Construction details for the
amplifier and DC offset circuit.Parts
are listed in Table 2.Photographs
illustrating the completed circuit and use of the circuit with the magnet and
coil assembly are shown in Figures 8 and 9.

Figure
8.Close-up view of completed amplifier
and DC offset circuit.Components and
jumper wires are placed in the "experimenter's board" using the plans
shown in Figure 7.

Figure
9.Photograph of amplifier circuit
connected to magnet and coil assembly to provide input signal and multimeter to
view output signal.A 6 volt battery is
used to provide power for the amplifier circuit.Alligator clip test leads are used to connect
the battery to the circuit and the output of the circuit to the
multimeter.It is useful to leave the
circuit uncovered and make the attachments with the test leads to illustrate
the use of electronic circuits.Circuits
that are similar in concept, but usually much more complicated, form the basis
for the many electronic products (computers, televisions, CD players, VCRs,
radios, etc.) that we use daily.